While not limited thereto in its utility, the present invention has applicability to the field of fiber optics. Liquid core fiber-optic waveguides, i.e., light guide fibers in the form of a capillary filled with a fluid which functions as the light transmitting core, have previously been proposed. For a first example of such a prior liquid core fiber-optic waveguide, reference may be had to U.S. Pat. No. 3,894,788. Since light cannot be efficiently propagated through a fluid filled capillary unless the refractive index of the capillary is less than that of the core fluid, the waveguides of U.S. Pat. No. 3,894,788 use an organic fluid as the core liquid. These organic fluids are specially selected so as to have refractive indices which are greater than that of the particular material from which the capillary is fabricated in order to permit long distance propagation of light waves through the core liquid.
A second example of a prior liquid core fiber-optic waveguide is an uncoated quartz or glass tube containing water. The interface between the outer surface of the tube and the air provides the "total reflection" surface which defines the waveguide. Such waveguides have limited practical application because the reflection surface is easily contaminated by contact with other components, finger prints or dust. In addition, the long tubes require thick tube walls due to the brittleness of the tube materials. Thick tube walls create a nonlinearity in light absorption spectroscopy because a higher proportion of the light travels in the tube wall rather than in the fluid core.
There has been a long standing desire to employ water or some other aqueous fluid in a liquid core fiber-optic environment for the purpose of facilitating chemical analyses of aqueous solutions by light interactive processes. A variety of techniques are available for use in the analysis of fluid samples. These techniques include optical methodology, particularly photometry and spectrophotometry, wherein the composition and concentration of dissolved substances are determined by measuring the absorption of light in a liquid which includes such substances. These optical analysis techniques are based on the fact that different substances will absorb light at different wave lengths. In the practice of these optical techniques, light absorption at discrete wave lengths or over a broad light spectrum, including ultraviolet, visible or infrared spectra, may be measured.
The need for instruments capable of the optical analysis of aqueous samples in the sub-milliliter volume range has grown in recent years. An important reason for this growing need is the fact that protein and DNA specimens are usually procured in small volume aqueous samples. For example, it is often difficult to obtain large amounts of animal, especially human, tissue samples for analysis. It is also costly to synthesize or purify protein, enzyme, antibody and DNA samples in large amounts.
Conventional absorption spectrometers are not sufficiently sensitive to analyze solutions prepared from the very small volume samples discussed above. For example, the approximate detection limit, defined as the lowest concentration that can be distinguished from background signal for double stranded DNA using absorption at a wave length of 260 nm is about 250 nanograms for a 0.5 ml, 10 mm light path length cuvette.
There have been efforts to reduce the requisite sample cuvette volume. Such efforts have often been characterized by a reduction in the light path length which, in turn, reduces instrument sensitivity. The smallest commercially available fluid sample cuvettes with 10 mm long light paths typically contain fluid volume in the 30 .mu.l to 50 .mu.l range. For a 5 .mu.l volume cell, however, the path length would be limited to 0.5 mm and thus unsatisfactory for analysis.
U.S. Pat. No. 5,416,879 which is assigned to the assignee of the present invention, discloses a liquid core fiber-optic waveguide to which the present invention relates. In a preferred embodiment, the waveguide is fabricated from an amorphous polymer material having a refractive index which is lower than that of water. Alternatively, the polymer material may be coated on the internal wall of a suitably prepared rigid tube comprised of glass or the like. The only currently available material having a refractive index less 1.33, which is chemically inert and insoluble in water, is a fluoropolymer, TEFLON AF.TM.. In certain applications this material has several shortcomings that may cause optical losses in the waveguide. The polymer is soft and may be easily scratched, adversely affecting its properties as a mirror surface. The polymer is also hydrophobic. Consequently air bubbles and less polarized molecules in aqueous solutions may stick to the surface of the tube.